The present invention pertains to the field of distillation of fluid mixtures, and in particular to distillation processes to separate multicomponent mixtures containing three or more components into at least four product streams each enriched in one of the components.
Consider the distillation of a multicomponent feed into multiple product streams each enriched in one of the components. Thermally coupled distillation column configurations are known to reduce total heat needed for the feed mixtures containing more than two components ("Minimum Energy Requirements of Thermally Coupled Distillation Systems," Z. Fidkowski, L. Krolikowski, AlChE Journal, pages 643-653, Volume 33, 1987). Specifically, for a ternary feed mixture it has been proven that a fully thermally coupled distillation configuration requires the lowest heat duty. However, conventional thermally coupled column arrangements require multiple distillation columns that lead to capital intensive processes.
A divided wall column arrangement for a ternary feed mixture is taught in U.S. Pat. No. 2,471,134 (Wright) as a means to reduce capital cost. As shown in FIG. 1, this prior art arrangement used only one column shell with one reboiler and one condenser. In FIG. 1, a ternary feed mixture 110 is fed to a distillation column 120. In this ternary feed mixture, A is the most volatile component and C is the least volatile component. From the top of the distillation column a vapor stream 140 rich in the most volatile component A is recovered. After condensation of this vapor stream in condenser 142, a portion is returned as liquid reflux stream 146 to the distillation column and the other portion is recovered as product stream 148. From the bottom of the distillation column a liquid stream 150 rich in the least volatile component C is collected. A portion of this liquid stream is collected as a product stream 158 and the rest of the portion is boiled in reboiler 152 and fed as vapor boilup stream 156 to the bottom of the distillation column. The divided wall 130 is located in the distillation column such that at the upper end u of the divided wall the descending liquid is composed primarily of A and B and is nearly free of the least volatile component C. Similarly, at the bottom end b of the divided wall, the ascending vapor is primarily composed of B and C and is nearly free of the most volatile component A. Therefore, it is possible to draw a B-rich product stream 168 from an intermediate location of the distillation column from the side of the wall that is opposite to the feed side.
Wright's divided wall column is an example of a distillation column that contains one partition and has two distillation zones. The primary distillation zone is the region containing the feed point of mixture 110 and the top and the bottom portions producing streams 140 and 150. The partition creates a secondary distillation zone producing B-enriched product stream 168. The liquid feed and the liquid reflux to the secondary distillation zone are provided directly from the primary distillation zone at the upper end u of the divided wall. Similarly, all of the vapor boilup at the bottom of the secondary distillation zone is provided from the primary distillation zone at the lower end b of the divided wall 130.
Wright's divided wall column is equivalent to a fully thermally coupled scheme and therefore requires low heat duty. Even though this scheme has been known for over 50 years, its use has been limited due to operational and control problems.
Recently, some attempts have been made to use Wright's divided wall column for some separations that are not very sensitive to liquid and vapor traffics in certain sections of the column. An example of such applications is in a paper by Lestak and Colllins ("Advanced Distillation Saves Energy & Capital", F. Lestak and C. Collins, Chemical Engineering, July 1997, pages 72-76). However, Abdul Mutalib and Smith found in their recent pilot plant studies that such a column arrangement is quite inflexible ("Operation and Control of Dividing Wall Distillation Columns, Part I: Degrees of Freedom and Dynamic Simulations", Trans IChemE, Vol. 76, Part A, page 308, 1998). It is difficult to control the descending liquid split on both of the sides of the divided wall at the top of the wall. Similarly, the vapor split on each side of the wall is difficult to control as it is affected by the liquid split. Generally, a distillation is sensitive to liquid to vapor flow ratios, and a lack of proper control of this ratio on each side of the divided wall makes Wright's divided wall column less attractive.
In the book entitled "Multicomponent Distillation--Theory and Practice", by Petluyuk and Cerifimow (page 198, FIG. VI-4e, published by Moscow Chemie, 1983), the authors disclose a configuration for a divided wall column where the partitioning wall is cylindrical and forms an annular or separation zone wherein the top and bottom communicate with the main distillation column.
Kaibel suggested a distillation with multiple divided walls to separate a feed mixture containing more than three components ("Distillation Columns with Vertical Partitions," G. Kaibel, Chem. Eng. Technol., Vol. 10, pages 92-98, 1987). Specifically, Kaibel showed arrangements to distill a four-component mixture and an eight-component mixture using divided walls with one reboiler and one condenser. His separation schemes used sharp splits between the components of intermediate volatility to distribute mixtures on various sides of the divided walls.
Recently, divided wall arrangements to separate a multicomponent mixture containing more than three components without any sharp split between the components of intermediate volatilities have been suggested by Christensen et al and by Agrawal ("Partitioned Petlyuk Arrangement For Quaternary Separations", A. C. Christiansen, S. Skogestad, and K. Lien, IChemE Symp. Series No. 142, pages 745-756, 1997; "More Operable Fully Thermally Coupled Distillation Column Configurations for Multicomponent Distillation", R. Agrawal, Trans. IChemE., Volume 77, Part A, pages 543-553, 1999). In these arrangements, a single distillation column with multiple internal partitions (walls) is used to produce all of the product streams, and the distillation column employs only one reboiler and one condenser. These arrangements suffer from the same problems of controlling the vapor and liquid flows as does Wright's column for a ternary feed.
FIG. 3 of the paper by Christensen et al. discloses the distillation of a four-component mixture where the liquid at the bottom of a secondary distillation zone producing a component of intermediate volatility (B) is boiled by heat exchange with vapor from the top of another secondary distillation zone producing another component of intermediate volatility (C). Such a heat transfer from one secondary distillation zone to another secondary distillation zone makes the independent control of liquid and vapor flows in each distillation zone extremely difficult. Operation of this type of distillation column would be quite difficult.
U.S. Pat. No. 5,946,942 (Wong, et al.) discloses an application of divided-wall principles to air separation. In this patent, an apparatus is described wherein the lower pressure column contains an inner annular wall. The region contained between the inner annular wall and the outer shell of the lower pressure column constitutes a secondary distillation zone that is used for the production of product argon. A drawback of this patent is that it does not teach a multizone distillation configuration to produce multiple product streams enriched in components of intermediate volatility.
The prior distillation schemes that require lower heat duty are quite complex and difficult to operate. As a result, many of these schemes lack operating flexibility and are rarely used in industry. Therefore, there is a need for distillation schemes that are easy to operate while having low heat requirements.
It is desired to have a more efficient and more operable multizone distillation process to distill a multicomponent feed stream into multiple product streams.
It is further desired to have multizone distillation processes that are easy to operate while having low heat requirements.
It also is desired to have multizone distillation processes which overcome the difficulties and disadvantages of the prior art to provide better and more advantageous results.